Electronic – Spikes, surges and noise sources in LTSpice

ltspicepspicespice

I am attempting to simulate a DC-DC power supply in LTSpice (using LT3748, 48V in, 5V 3A out).

There is concern about the amount of noise that will be on the 48V input rail, and so we want to simulate with a noisy input (exact figures on the noise levels are also part of my research, but that will be a different question).

To simulate the noise on the power rail I started with something similar to the answer given to this question:
How do you simulate voltage noise with LTSpice?
Using two voltage sources to give the power, separated from each other with resistors:
Two input voltages, each leading into 100R resistors before power rail

This then gives a noisy output based on what SIG and NOISE are set to. My question is; if I know what I want to have on the power rail, how do I set the voltages of NOISE to give me the spike etc that I want? The current draw will clearly play a part, but that is part of what I'm trying to measure. Is there a way to work out what my voltage spikes on V2 should be to get a 200V surge spike on the IN rail based on the value of R9 and R8?

I should add that the NOISE simulation in LTSpice is not really what I am after, I am looking more at surges and spike than noise. There is also an issue with the model of LT3748 (and error about it being time dependant is raised), and so the voltages on OUT are not correctly modelled.

(While this is an isolated supply, I haven't isolated the grounds on either side just to make it that bit quicker to draw).

EDIT: I realise that when I've been saying "noise", that has been wrong. I should have used "surges" and "transients", as they are a better description of what I'm interested in. So the question would be better phrased "how do I put a surge onto my input rail?". With that as the question, PlasmaHH probably has a good answer; putting two supplies in series with one another so that my stable 48V has the pulses put on top of it.

Best Answer

I concur with @PlasmaHH's idea for the usage of PWL. If you only need a few data points, use the PWL source type directly. Otherwise, put the values into a text file and feed that to the PWL. Excel is nice for this, export as .csv file. That way, any kind of data you can imagine can be converted to a voltage.

This can also work for other primitives besides voltage. Say you wanted a wildly-varying resistance:

  • Create a new voltage source, say V3. Ground one end of it.
  • Create a new net label, say V3val, and wire it to the V3 source.
  • Put the data points into the PWL file of V3 (use whole integers, not "10k".)
  • Add a resistor say R5, and change it's "R" value to "R=V(V3val)".

Then R5's resistance will be modeled as the "voltage" generated by V3.

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